1. Field of the Invention
The present invention relates in general to electric-component mounting system and method for mounting electric components (typically, electronic components) on a circuit substrate, and more particularly to techniques for improving the efficiency of mounting of the electric components.
2. Discussion of Related Art
JP-A-342998 discloses an example of an electric-component mounting system including a plurality of component-holding heads which are arranged around a common axis of turning and turned about this common axis, to mount electric components on a printed-wiring board. The component-holding heads are disposed on an indexing body rotatable about a vertical axis, such that the component-holding heads are equiangularly spaced apart from each other along a circle having a center on the vertical axis of rotation of the indexing body. With a rotary intermittent motion of the indexing body, the component-holding heads are turned about the vertical axis of rotation of the indexing body (which is the above-indicated common axis of turning of the heads). The indexing body has a plurality of working positions or stations at which the component-holding heads are temporarily stopped. These working positions includes a component-receiving position and a component-mounting position. At the component-receiving position, the component-holding head receives an electric component from a component supply device. At the component-mounting position, the electric component is transferred from the component-holding head onto the printed-wiring board.
In this electric-component mounting system, a printed-wiring-board supporting and positioning device is provided to support the printed-wiring board and move the board in a horizontal plane parallel to the component-mounting surface of the printed-wiring board. The printed-wiring board is positioned such that a selected spot on the component-mounting surface of the board is located right below the component-holding head stopped at the component-mounting position, so that the electric component is mounted at the selected spot. The printed-wiring board is moved during rotation of the indexing body, and is stopped when the electric component is mounted on the board. Thus, an operation to mount the electric component on the board is performed while the component-holding head and the printed-wiring board are both stopped. However, the electric-component mounting system may suffer from a deviation of the position of the electric component actually mounted on the board, with respect to a nominal position on the board. This deviation of the actual mounting position of the component from the nominal position may be caused by an error of relative positioning between the component-holding head and the printed-wiring board, a positioning error of the electric component as held by the component-holding head, and a positioning error of the board by the printed-wiring-board supporting and positioning device.
The different component-holding heads have respective different positioning errors relative to the printed-wiring-board supporting and positioning device, and each component-holding head has substantially the same positioning error relative to the printed-wiring-board supporting and positioning device, for all of different kinds of electric components and for all of different mounting positions on the board. In view of these facts, it is a conventional practice to obtain the amount and direction of positioning error of each of the plurality of component-holding heads, and board-positioning data to position the printed-wiring board upon mounting of the electric components on the board are compensated on the basis of the obtained amount and direction of the positioning error, so that the electric components are mounted at the respective nominal positions. Thus, the position of the board upon mounting of each electric component is adjusted to reduce or eliminate the positioning error of each component with respect to the nominal mounting position.
The present inventors attempted to increase the acceleration and deceleration of the component-holding heads during movements of the heads, in an effort to reduce the required time of the movements for thereby improving the efficiency of mounting of the electric components. However, the increased acceleration and deceleration of the component-holding heads resulted in a considerable amount of deviation of the actual mounting positions of the electric components with respect to the nominal mounting positions, in spite of the compensation of the board-positioning data on the basis of the obtained amount and direction of the positioning error of each component-holding head. It was found that the amount and direction of the positioning error of a given component-holding head which is obtained during a movement of the head at a speed controlled in a certain pattern do not permit elimination or sufficient reduction of the positioning error of the same component-holding head when the head is moved at a speed controlled in another pattern. In this case, the mounting accuracy of the electric component is deteriorated. Thus, there is a limitation in the degree of improvement of the component mounting efficiency by reducing the required time of movement of the component-holding head, while assuring a sufficiently high degree of component mounting accuracy. This limitation appears to arise from an increased amount of vibration of the component-holding head caused by the increased acceleration and deceleration of the head, due to insufficient rigidity of the component-holding head and a device including the indexing body for turning the component-holding head. Namely, the electric component appears to be mounted on the printed-wiring board before the vibration has been sufficiently attenuated after the head is stopped. Although it is considered to improve the component mounting accuracy by increasing the rigidity of the component-holding head and the turning device, an increase in the rigidity necessarily causes an increase in the masses of the head and the turning device, which in turn causes problems such as deterioration of the component mounting accuracy, and an increase in the cost of manufacture of the electric-component mounting system.